A. Field of Invention
The present invention relates generally to expansion sealing plugs and more particularly to a new and improved tapered expansion plug for sealing access and other bores in metal castings, metal forgings and other metal parts as well as plastic parts.
B. Description of Related Art
Prior art expansion plugs of the type disclosed in the U.S. Pat. No. 2,821,323 of Leighton Lee, II, granted Jan. 28, 1958 and entitled "Pin Plug" have been designed for sealing hydraulic systems. Increased hydraulic system pressures have required utilizing materials of substantially higher strength and hardness, which in turn required an improved expansion plug as disclosed in U.S. Pat. No. 4,867,333 to Kolp, Anderson and Dickey granted Sep. 1989 entitled "High Pressure Pin Plug" which improvements provide for sealing of hydraulic systems exposed to higher ranges of system pressures. Both said prior art plugs function by the controlled expansion of a cylindrical plug member by the forcible insertion of a tapered pin into a central, axially extending bore in the plug member, thereby expanding the plug walls to engage the installation bore. Similarly, the expansion plug disclosed in U.S. Pat. No. 3,825,146 to Hirmann, granted Jul. 23, 1974 involves the use of an inner expansion member, in this case a ball, that is forced into a bore of lesser diameter. Said plugs require a shoulder within the installation bore to prevent the plug from moving during insertion of the expansion member. For these reasons, installation of the prior art plugs require the reaming of an installation bore of greater inside diameter than the original opening. Due to the relatively small size of the plugs, minor variations in tooling or reaming technique can produce clearances between the outside surface of the plug and the inside surface of installation bore that are significant to the function of the plug. Therefore, in said prior art expansion plugs, an installation tolerance must be allowed between the installation bore and the unexpanded plug outer surface. During installation of a prior art expansion plug, a portion of the radial expansion of the plug may take place before engagement with the installation bore as the clearance is being bridged. The prior art plug members are of uniform outside diameter which must not exceed the inside diameter of the installation bore. At the same time, there is a maximum clearance between the plug walls and the installation bore that can be bridged by the plug expansion without unacceptably compromising the function of the plug. The specified performance level, i.e. the blowout pressure of the plug, depends upon the radial force exerted against the installation bore which in turn depends upon the amount of expansion of the plug after the clearance between the plug outer surface and the installation bore has been bridged. Therefore, the total plug expansion must be increased by adding the installation tolerance to the expansion required for specified sealing performance. As an example, if, given particular materials, bore size and other variables, a minimum radial expansion of eight-thousandths of an inch is required while a two-thousandths of an inch radial tolerance must be allowed for variations in the diameter of the installation bore, the total expansion of the plug must equal the sum of the minimum expansion and the installation tolerance, in this case, ten thousandths of an inch. It is to be expected that some installations will result in a perfect fit and as a result, the component must tolerate the total radial expansion (0.010 inches in the example). The anticipation of occasional total radial expansions greater than that required for sealing must therefore be considered in the design of the component that includes the installation bore. Frequently the installation bore is formed in a boss designed to absorb the stress of the expansion. Generally, greater expansion requires a boss of larger diameter that would therefore be heavier. A component that must be heavier than is otherwise required is generally considered to be a disadvantage, particularly in aerospace applications. The additional expansion needed in the prior art plugs to absorb the installation tolerance has a number of adverse consequences in addition to the consideration of the installation environment. In particular, in the plugs utilizing pins as the expansion members, it has been found necessary to put a wax coating on the pin to aid installation. In addition, preassembly of the pin and plug members has not been found to be practical, in part because of the large diameter of the pin and in part because of the radial distortion of the plug body caused by preinsertion. Because the pin is not inserted into the plug under controlled conditions, a vent bore must be provided through the pin to allow the escape of any matter that would otherwise prevent or hinder installation. The wax coating and vent bore increase the manufacturing costs of the prior art plugs and the assembly required makes installation more difficult than would be the case for an one piece plug.
In addition, in practice, in circumstances where there is insufficient or negative clearance between the installation bore and the outside diameter of the plug of the prior art, the plug may be forced into the installation bore damaging the sealing surfaces. Further, the prior art expansion plugs, because of the importance of minimizing the bore-plug clearance, must be manufactured and stocked in numerous sizes to match the size of the bores. Even with the availability of a variety of sizes, installation must be relatively precise to assure proper functioning of the plug.